The recent development of office automation has demanded various terminals. Inter alia, recording devices for converting electrical signals to visual images, so-called printers, enjoy an increasing demand, but a few of the conventional recording devices are satisfactory in performances. Currently employed recording systems include an ink jet system, an electrophotographic system, a heat transfer system, and the like. However, use of a liquid ink or a powder, e.g., toner, makes maintenance and operation of the devices complicated, or a thermal head used has a short life time or achieves only a low printing speed.
An electrical discharge transfer recording technique is known to be one of means for forming images having a relatively high resolving power. In this connection, Japanese Patent Publication No. 19819/70 discloses a thermographic copying process, and Japanese Patent Publication No. 22030/82 discloses a transfer medium.
The conventional electrical discharge transfer technique will be described below with reference to the accompanying drawings.
FIG. 1 illustrates a cross section of the conventional electrical discharge transfer medium, in which light reflecting layer 2 is provided on support 1 and light-heat converting layer 3 and heat transfer solid ink layer 4 are provided in this order on the reverse side of the support 1. A surface roughening layer (not shown) may be provided between the support 1 and the light reflecting layer 2 to facilitate and stabilize destruction of the light reflecting layer 2 upon electrical discharge.
FIGS. 2 to 4 each shows a recording process by the use of the recording medium of FIG. 1. In these figures, numerals 5, 6, and 7 indicate an image-receiving sheet, a xenon lamp, and a flash light, respectively, and other have the same meanings as in FIG. 1. In carrying out recording, the light reflecting layer 2 is removed in accordance with an information pattern to be recorded by a well-known discharge destruction technique as shown in FIG. 2. The image-receiving sheet 5 is intimately contacted with the heat transfer solid ink layer 4, and the flash light 7 containing ultraviolet rays, visible rays, and infrared rays emitted from the xenon flash lamp 6 is irradiated on the light reflecting layer 2 as shown in FIG. 3. The flash light 7 irradiated on areas where the light reflecting layer 2 remains is reflected, while that on areas where the light reflecting layer 2 has been removed passes through the support 1 and reaches the light-heat converting layer 3, where the flash energy is absorbed and effectively converted to a heat energy. The heat transfer solid ink 4 on the light-heat converting layer 3 is thereby fused or sublimated by the heat energy and transferred and fixed onto the image-receiving sheet 5 to obtain transferred image 8 as shown in FIG. 4-(a).
Further, IBM Technical Disclosure Bulletin, Vol. 18, No. 12, 4142 (1976, May) discloses a thermal laser transfer printing process. This process comprises converting a laser beam based on an image information on an ink sheet comprising a support having provided thereon a heat transfer solid ink layer and converting the laser light energy to a heat energy by the action of the ink, to thereby imagewise transfer and fix the ink to an image-receiving sheet disposed in intimate contact with the heat transfer solid ink layer, similarly to the electrical discharge transfer technique.
The above-described conventional electrical discharge transfer techniques succeeded to obtain a relatively clear image having a desired density and substantial faithfulness to an original by the discharge destruction recording when an image-receiving sheet has a high surface smoothness as shown in FIG. 4-(a). However, when an image-receiving sheet of low surface smoothness, such as commonly employed papers, e.g., copying paper, and bond paper for business use, is used, the ink transfer is restricted to contact points between the ink layer and the image-receiving sheet and their vicinities as shown in FIG. 4-(b), resulting in a failure of transfer of a solid image or a fine line image.
Transferred image quality might be improved by lowering the melting point or melt viscosity of a heat-fusible binder or lowering the temperature at which a subliming coating starts to sublime. Such attempts, however, cause unresolved transfer called bridging phenomenon or unnecessary transfer at relatively low temperatures, leading to reduction in preservability and background stains (fog).
A great feature of the electrical discharge transfer system resides in faithfulness and sharpness of transferred characters or images at high resolving power. However, images obtained by the use of the aforesaid conventional transfer media often have fat edges due to smearing or blur and are, therefore, inferior in image quality such as contrast or sharpness.
In full color recording, it is required to achieve tone reproduction of each primary color. However, the conventional electrical discharge transfer media involves a difficulty in faithfully transferring the tone obtained by discharge destruction. In some detail, when tone reproduction is effected by a variable area method, such as a dither method, in forming a pattern by electrical discharge, the irradiation area of a flash energy to be absorbed in an ink layer or a light heat converting layer can be controlled in agreement with a dot density to be recorded. Nevertheless, sufficient tone reproduction cannot be achieved due to poor definition upon transfer. That is, a transfer recorded density tends to be saturated at a given level, failing to realize tone reproduction at high density.
Similarly to the electrical discharge transfer system, the heat transfer system making use of a laser beam has a problem of poor ink transfer properties onto an image-receiving sheet having a low surface smoothness and, therefore, inevitably requires papers having high surface smoothness, which naturally leads to an increased printing cost. In this system, the ink transfer properties to an image-receiving sheet of low surface smoothness might be improved by raising the laser beam energy or increasing contact pressure between the ink sheet and the image-receiving sheet, but such makes a recording device large-sized and expensive.